This invention relates to packaged microelectronic devices, and more particularly to finishing or refurbishing surfaces on protective covers encapsulating microelectronic dies.
Microelectronic devices, such as semiconductor devices, are used in products ranging from personal computers to automobiles. In most applications, microelectronic devices have a microelectronic die, a lead frame or printed circuit substrate attached to the die, and a cover or xe2x80x9cpackagexe2x80x9d encasing the die. The packages protect the die from physical damage, particulate contamination, electrical interference and abuse. The packages also provide environmental protection from chemicals, moisture, and gases that may impair the functionality of the devices. An additional function of the protective packages is to dissipate heat generated by the microelectronic devices during operation.
Microelectronic device packages are typically composed of epoxy resins or polyimide materials that offer the advantages of low weight, low material cost, and manufacturing efficiency. These xe2x80x9cplasticxe2x80x9d packages are produced by epoxy molding in which epoxy in a flowable state is forced into a mold to create an individual package around a microelectronic die. Multiple packages can be produced simultaneously by arranging the microelectronic devices on a plurality of lead frames, and positioning the lead frames between two mold halves having cavities around each individual die. A ram forces a flowable epoxy into each cavity to form the individual packages. After the epoxy sets in the mold, the mold halves are separated and the lead frames are placed in an oven for final curing of the molded packages. A precision cutting tool is used to separate the lead frames into individual packages after curing.
It is often desirable for microelectronic device packages to have a uniform surface finish that reflects a quality product and provides an adequate marking surface. The completed package is usually marked with key part information, such as the product type, device specifications, date, and lot number. The packages are also marked to provide fiducial reference points for optically aligning the packaged devices in subsequent processing by the device manufacturer or for handling by Printed Circuit Assembly (PCA) manufacturers. A common method of marking packages is to apply an ink with an offset printer followed by a curing step.
One problem associated with molded packages is that the surface finish may not provide an adequate marking surface. For example, resins can build up in the mold cavities after a period of use and leave asperities or stains on the molded surfaces of the package. When this happens, the molds must be cleaned and the packages may have a poor quality surface finish. As such, resin buildup in the mold cavities can reduce the throughput of molding machines and the yield of finished devices. One conventional solution to resolve resin buildup is to use a release film in the mold that seals the mold and reduces the frequency of cleaning. These films, however, can wrinkle and produce asperities or other marks on the molded surfaces of the package.
A poor surface finish on the plastic packaging can lead to many other problems. For example, stains and asperities on a package marking surface can lead to poor ink adhesion that can obscure fiducial marks and part information. It will be appreciated that poor quality fiducial marks on the surface of the package can lead to imprecise operation of the machine vision equipment used in subsequent processing and handling of the packaged devices. Moreover, if the key part information identified on the package is significantly obscured or unreadable, the packaged device may be rejected. In any case, poor quality marking cosmetically reflects poor workmanship. Accordingly, it would be desirable to develop a method of producing an adequate marking surface on a molded microelectronic device package that does not require frequent cleaning of the molds.
The present invention is directed toward methods for finishing surfaces on protective packages encapsulating microelectronic dies. In one embodiment, the method includes abrading a surface of the package by engaging an abrasive media with the surface of the package, and stopping the abrasion when a surface blemish has been at least partially removed from the package. In one aspect of this embodiment, the abrasive media can be a fixed-abrasive member, and abrading the surface of the package can include pressing the surface of the package against the fixed-abrasive member and imparting motion to the package and/or the member to rub the package surface against the member. The fixed-abrasive abrasive member can also be used with a solution, and abrading the surface of the package can include rubbing the package surface against the fixed-abrasive member in the presence of the solution. In another aspect of this embodiment, the abrasive media can be a non-abrasive member and a solution having abrasive particles, and abrading the surface of the package can include rubbing the package surface against the non-abrasive member in the presence of the solution having abrasive particles. In yet another aspect of this embodiment, the abrasive media can be an abrasive blasting media, and abrading the surface of the package can include driving the abrasive media against the surface of the package.
The methods for finishing a surface of a protective package by abrasion can include controlling the depth of the abrasion by determining a depth at which the abrasion will have removed sufficient blemishes in the surface to attain a preselected surface finish, and terminating the abrasion at the depth when the preselected surface finish has been attained. The methods can also include cleaning residual materials from the package after terminating the abrasion of the package surface.
In another embodiment of the invention, the method for finishing a surface of a protective package on a microelectronic device includes chemically etching a surface of the package to remove a layer of material from the package, and cleaning residual materials and/or chemicals from the package after terminating the etching of the package surface. In one aspect of this embodiment the method can include controlling the depth of the etching by determining a depth at which the chemical etching will have sufficiently removed the blemishes from the package surface to attain a preselected surface finish, and terminating the chemical etching at the depth where the preselected surface finish has been attained.
In yet another embodiment of the invention, the method for finishing a surface of a protective package on a microelectronic device includes ablating the surface of the package to remove a layer of material from the package, and cleaning residual materials from the package after terminating the ablation of the package surface. In one aspect of this embodiment, ablating the surface can include ablating the surface of the package with laser light.
Another method for finishing the surface of a protective package on a microelectronic device in accordance with the present invention includes pressing a surface of a stamp or press against the package surface to emboss the package surface, controlling the embossing by determining the time required to produce a preselected surface finish, and terminating the embossing after the time required to produce the preselected surface finish has elapsed. In one aspect of this embodiment, the method can include heating the surface of the stamp or press while it presses against the package surface.